1. Field of the Invention
This invention relates to the separation of liquids by crystallization. More particularly, this invention relates to the cooling of a liquid containing a crystallizable material by means of a volatile liquid. This invention especially relates to the direct cooling of a liquid containing a crystallizable material by the controlled vaporization of a volatile liquid.
2. Description of the Prior Art
Crystallization to recover such basic materials as salt and sugar has been practiced for centuries. Voluminous quantities of crystalline substances are produced commercially each year. Sodium chloride, sodium and ammonium sulfates and sucrose are produced worldwide in excess of 100 million tons per year. Many of the fine organic chemicals and pharmaceutical chemicals are produced in crystalline form. Crystallization is often employed as an alternate to fractional distillation to separate azeotropes and close boiling mixtures. For example, para-xylene is separated by crystallization from equilibrium mixtures of its isomers, meta and ortho xylene and other C.sub.8 aromatic hydrocarbons. In addition, crystallization separations are often attractive because the heat of crystallization is usually significantly lower than the comparable heat of vaporization and the crystallization can be conducted nearer to ambient temperatures than distillation.
Crystallization normally involves the removal of both sensible and latent heat to produce the crystalline product. Indirect heat transfer has been employed in crystallization processes heretofore but crystal formation on the surface of the coils, jackets and heat exchanger tubes reduces the effectiveness of the heat transfer surface. Scraped-surface crystallizers have also been employed in the form of jacketed vessels and double-pipe heat exchangers but maintenance of the equipment can be costly and time-consuming.
Direct contact cooling avoids many of the problems of the scraped-surface crystallizers. However, although direct contact cooling provides improved heat transfer and eliminates the crystal formation on heat transfer surfaces, product contamination and recovery of the coolant can cause problems. In this type of cooling, the coolant may be a solid, a liquid or a gas and it may transfer sensible and/or latent heat. Thus the coolant may or not evaporate during use and it may be miscible or immiscible with the mother liquor.
The evaporation of a light hydrocarbon to provide sufficient cooling for crystallizing a crystallizable material from a hydrocarbon mixture is well known in the petroleum industry in such processes as the dewaxing of lubrication oils and the separation of para-xylene from its isomers. U.S. Pat. No. 2,614,065 of Wanderer et al. and U.S. Pat. No. 3,443,391 of Storment et al. disclose the propane dewaxing of lubricating oil wherein propane is employed as a direct contact refrigerant to crystallize wax from lube oil feedstocks. U.S. Pat. No. 3,859,811 of Duncan discloses the use of direct contact Freon refrigerants to crystallize para-xylene from a mixture of para and meta xylene and the use of ethane, ethylene or Freon refrigerants to dewax oil in a direct contact cooling process. After evaporation, the direct contact refrigerants are condensed by means of a closed loop refrigeration system and returned for reuse.
The recovery of durene by crystallization from a mixture of hydrocarbons is disclosed in U.S. Pat. No. 2,560,373 of Shmidl, U.S. Pat. No. 2,815,392 of Bown, U.S. Pat. No. 2,914,586 of Walker and U.S. Pat. No. 3,103,541 of Smith et al. Although the processes disclosed in these patents employ indirect cooling means or unspecified cooling means to cause durene to crystallize from a hydrocarbon mixture, none of them provide details of the refrigerant or its use. All of these patents are concerned with either the recovery of durene in a high degree of purity or with improvements in the crystallization operation itself.
It is an object of this invention to provide a crystallization process employing direct contact cooling.
It is another object of this invention to provide a crystallization process wherein the direct contact refrigerant is employed substantially at room temperature and at slightly above atmospheric pressure.
It is yet another object of this invention to provide a crystallization process which minimizes energy consumption and pollution.
It is a further object of this invention to provide a crystallization process which requires minimum equipment and investment costs.
It is another object of this invention to provide a process for the crystallization of durene from a hydrocarbon mixture by means of a direct contact refrigerant.
The achievement of these and other objects will be apparent from the following description of the subject invention.